1. Field of the Invention
This invention relates to a novel magnetostrictive pumps capable of delivering precise quantities of fluids. In particular, a magnetostrictive pump adapted for fluid delivery for high performance liquid chromatography is described.
2. Description of the Related Art
Magnetostrictive materials are those which are deformed under the influence of a magnetic field. Many magnetostrictive materials are known such as those described in European Patent Appl. No. 92300980.7, which is incorporated herein by reference. A magnetostrictive material which is commercially available is Terfenol-D.RTM.,which is an alloy containing iron and the rare earth metals terbium and dysprosium, and which has the stoichiometric composition, EQU Tb.sub.x Dy.sub.1-x Fe.sub.1.9-1.98
This material is available from Etrema Corporation, Ames, Iowa. Terfenol-D.RTM. typically is characterized by a magnetostrictive effect which is two orders of magnitude greater than materials which are conventional and not "giant" magnetostrictive such as Terfenol-D.RTM..
At the present time, magnetostrictive pumps exist which are utilized to pump fluids. U.S. Pat. No. 4,726,741 discloses a magnetostrictive pump including a magnetostrictive element in direct communication with a portion of a pump cylinder containing a fluid to be pumped. Magnetically controlled valves are utilized to control the direction of fluid flow in response to movement of the magnetostrictive material. Sealing means such as "O" rings are provided between the magnetostrictive material and the housing for the magnetostrictive material.
U.S. Pat. Nos. 4,795,317 and 4,795,318 disclose magnetostrictive pumps wherein the magnetostrictive element directly contacts a fluid within a portion of a pump cylinder which fluid is to be pumped under the influence of the moving magnetostrictive material. These pumps also include a sealing means between the magnetostrictive material and the housing for the magnetostrictive material.
German patent application DE 4204435 A1 discloses a magnetostrictive pump utilizing a magnetostrictive element which is in direct contact with a flexible membrane which, in turn, contacts a fluid to be pumped. German patent application DE 4032555 A1 discloses a magnetostrictive pump utilizing a magnetostrictive element which directly contacts a diaphragm in the shape of a bellows. The bellows encloses a fluid to be pumped under the force exerted by the magnetostrictive element.
Magnetostrictive pumps utilizing a magnetostrictive element which directly contacts a fluid to be pumped are undesirable since such pumps require a sealing means between the magnetostrictive material and the housing containing the magnetostrictive material in order to prevent fluid by-pass between the magnetostrictive element and the housing. These seals rapidly deteriorate under the force exerted by the reciprocating magnetostrictive element. Magnetostrictive pumps constructed so that the magnetostrictive element directly contacts a diaphragm are undesirable since the materials normally utilized to form the diaphragm have limited mechanical strength and fail within a relatively short time due to fatigue and corrosion.
High performance liquid chromatography (HPLC) is a process wherein a sample is separated into its individual constituents by passing the sample through a column of material, usually particles, which functions to separate the sample constituents within the column. The sample is delivered to the column with a pump which has a very stable flow rate. Pressures, between about 50 psi and 6000 psi are common with a flow rate typically between 1 microliters per minute and 5 milliliters per minute. This flow rate is usually programmable and is as free of pulsations as possible. At the present time, there is no available pumping system which can achieve the full range of these pressures and flow rates with a single pump design that satisfies the stability and accuracy requirements.
Conventional liquid chromatography (LC) pumps typically are based on the use of reciprocating pistons driven by means of electric motors (usually stepping motors) leveraged through a gear train or cams. The forces inherent in the electric motors are modest, and extensive gearing is essential in order to attain the desired high pressures. Generally, two identical reciprocating pump heads are required for steady flow. The pump heads are run out of phase with each other so that one is pumping liquid while the other is refilling with liquid to be pumped. In the motor-driven pumps, the switchover between heads usually results in pressure transients of approximately 300 psi. Furthermore, seals surrounding the sliding pistons reduce the reliability of reciprocating pumps because they wear and occasionally shed particles of material into the sample fluid stream, causing malfunctions of check valves and a requirement for larger capillary tubing within the system to prevent clogging due to the particles.
Another category of presently available pumps are diaphragm pumps. These pumps have flexible "bellows" driven with electric motors by a mechanism of cams, and have the same problems of pulsation as the reciprocating pumps. These diaphragm pumps operate with a shorter stroke than reciprocating pumps, and those presently available do not attain the high pressures provided by reciprocating piston pumps due to the lack of available force of present drive mechanisms. However, diaphragm pumps are generally regarded as being more reliable, since they do not require the seals used by piston pumps.
Accordingly, it would highly desirable to provide a magnetostrictive pump and pump system which does not require seals between adjacent moving and static parts, and which is capable of generating high pressure within a wide pressure range, and maintaining precise flow rates. Furthermore, it would be desirable to provide such a pump and pump system which is capable of reliably withstanding the force and pressure changes normally encountered in HPLC procedures for long periods of reliable operation.